I have long assumed that when a CME (aka solar mass ejection, SME) hits a spacecraft, its particles will be coming in a straight line from the Sun. I've learned recently that it isn't so direct, and many models assume an isotropic model, meaning that the particles come from all directions at the same time.

Although the magnetic field lines of the Sun bend the particles' paths, a purely isotropic distribution seems odd. Is directional shielding sufficient for CME protection for a spacecraft in interplanetary space?

$\begingroup$I really would like to know from which source you learned that the particles come from all directions at the same time. This could be so within the corona, but i just can't imagine this would be the case at the distance of Mercury, for instance. You think of a beam of plasma, with a diameter of millions of km, stretching thens of millions of km in a partical direction ?$\endgroup$
– CornelisinspaceAug 11 '19 at 11:32

$\begingroup$It started from a comment I saw on a different website. I've been having difficulty getting anything remotely reliable, but from what I can tell it will be a broad, but still very directional impact. There are a lot of small bits and pieces of papers that reference assumed isotropic distribution of particles, but there isn't a great source that I have found that shows things one way or another.$\endgroup$
– PearsonArtPhoto♦Aug 11 '19 at 11:57

2 Answers
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A single CME will impact a spacecraft from only one direction, but that direction might not be directly from the sun because a CME may zigzag en route. So unless you can know in advance from which direction it's coming, a shield just large enough to block direct solar radiation won't suffice.

The farther the spacecraft is from the sun, the smaller (and thinner) the shield you can probably get away with, though: full sphere, hemisphere, small disc.

The more technical article from 2010 in Nature that it references Propagation of an Earth-directed coronal mass ejection in three dimensions (also in ArXiv) talks about nonradial velocities and deflections based on modeling and observations from the STEREO pair of spacecraft, and uses math that's beyond me. My takeaway is that space weather is tricky, just like atmospheric weather or Mars weather.

Particles that come from all directions at the same time can only occur within an area of gas or plasma, the corona in this case, with a certain density.

At a certain distance outside the corona the density has become so low that the particles of the plasma don't bounce at each other anymore to change in direction and so for one point outside that greater area of plasma around the corona the particles can come only from the direction of that area.

At the distance of Mercury from the Sun the density of the plasma will certainly be that low that no bouncing at each other of particles will occur, so for interplanetary spacecraft the plasma will certainly come from one direction, i.e. the direction of the Sun.